For the purpose of increasing the transmission capacity of a wavelength division multiplexing optical transmission system, a method of narrowing the intervals of multiplexing wavelengths to increase the number of wavelengths for transmission use and a method of increasing the bitrate have been known. For example, consider the case of doubling the transmission capacity of a system that multiplexes signals having a bitrate of 10 Gb/s with 32 wavelengths at intervals of 50 GHz. According to the former method, the wavelength multiplexing portions are exchanged to narrow the wavelength intervals down to 25 GHz for 64-wavelength multiplexing. According to the latter method, the transmission capacity of the foregoing system can be doubled by making the bitrate of the signal to be carried by a wavelength twice, i.e., 20 Gb/s.
In one example of the latter method, two 10-Gb/s signals (signals having a bitrate of 10 Gb/s) are transmitted on a wavelength at 20 Gb/s by using Differential Quadrature Phase Shift Keying (DQPSK) for four-phase differential phase modulation. For example, according to the method described in Non Patent Literature 1 listed below, two optical transmission and reception modules (XFPs: 10 Gigabit Small Form Factor Pluggables) receive respective two different STM (Synchronous Transfer Mode)-64 signals (with a data rate of 10 Gb/s). Such two signals will be referred to as tributary signals. Then, an error correction LSI (Large Scale Integration) encodes each of the tributary signals, and a differential coder composed of a parallel prefix network encodes the signals further for electrical multiplexing. A DQPSK modulator modulates the electrically-multiplexed signal into a 20G DQPSK signal of 12.4 Gsymbol/s (=24.8 Gb/s) and sends it out.
When receiving a 20G DQPSK signal that is sent thus, a 20G DQPSK receiver composed of two one-bit delay interferometers in parallel extracts two electrical tributary signals from the received signal. The error correction LSI decodes the tributary signals, and the optical transmission modules (XFPs) send out the respective tributary signals decoded. Here, the 20G DQPSK receiver equally splits the 20G DQPSK signal through an optical coupler before the two one-bit delay interferometers convert information on phase differences with adjacent bits into intensity information, and differential optical receivers optically receive the respective intensities. The two one-bit delay interferometers decode respective different data signals with an optical phase of π/4 and −π/4, respectively.
The one-bit delay interferometers mentioned above require high optical phase accuracy and stability, whereas the optical phase may have errors due to internal factors (such as a change in composition) and external factors (such as a change in temperature and pressure). Phase stabilization control is therefore needed. In the phase stabilization control, it is not fixed which of the two tributary signals is decoded by which of the one-bit delay interferometers, nor is the logic (positive logic, inverted logic) fixed. To solve such a problem, according to the following Patent Literature 1, for example, a logic-tributary decision circuit is used to allow selection of the output destinations and logic of the two tributary signals.